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IEEE 802.11 Wireless LAN Standard
Physical Layers and Security Updated: 5/6/2011 Reading Assignments: Chapter 14, Walsh Codes pp.184, OFDM pp.337,
Example
Example Beacon: locate the BSS ID Contention Free Period CF-Poll: AP->STA (got something)
802.11 Physical Layer o Issued in four stages
Stage 1
Stage 2
Stage 3
Stage 4
1Mbps/2Mbps operating at a wavelength between 850-950 nm
802.11 Physical Layer
Physical Media Defined by Original 802.11 Standard o Direct-sequence spread spectrum
n Operating in 2.4 GHz ISM band n Data rates of 1 and 2 Mbps n Uses Barker Sequence (11-bit chip)
1à +-++-+++--- 0à -+--+---+++
Autocorrelation of Barker Sequence (11-bit chip)
o For N=11 o For R(τ =0)=11/11=1
o For R(3)=-1/11
o Same for R(1) = R(N-1 =10)=-1/11
We want |R(τ)|<=1 for all |τ|<=N-1; N is the number of chips in the code
Autocorrelation of the code sequence
Note(τ) represents shift from correct value! (τ) = 0 indicates the received value is correct
Physical Media Defined by Original 802.11 Standard
o Frequency-hopping spread spectrum n Based on signal hopping
concept o 2.5 hop / sec with hop
distance of 6 MHz n Operating in 2.4 GHz ISM band
(unlicensed) n Data rates of 1 and 2 Mbps
o Infrared n Omni directional with 20 meter
of range n 1 and 2 Mbps n Wavelength between 850 and
950 nm
2/4 Level Gaussian FSK/
802.11b
o Extension of 802.11 DSSS o Provides data rate at 5.5 and 11 MHz o Same chipping rate but higher data
rate using Complementary Code Keying (CCK) modulation.
Using 8x8 Walsh Matrix
6 bit
1 bit QPSK
Walsh Matrix o Orthogonal Codes used in CDMA applications o Walsh codes of length n consists of n rows of an nxn Walsh
Matrix o Properties
n Every row is orthogonal to every other row n Every row is orthogonal to the logical NOT of every other row
8x8 Walsh Code
A single Walsh code
W(8,2)
Walsh code orthogonality o Code is given as a row in WC matrix o To generate a code
n “0” -> “1” n “1” -> “-1”
o Example: Codes W4,2 and W4,3 n W8,2 : (0,0,1,1,0,0,1,1) -> (1,1,-1,-1,1,1,-1,-1) n W8,3 : (0,1,1,0,0,1,1,0) -> (1,-1,-1,1,1,-1,-1,1)
0)1,1,1,1,1,1,1,1()1,1,1,1,1,1,1,1(WW 8,38,2 =−−−−⋅−−−−=⋅
When synchronized – codes are orthogonal
When out of sync – codes are not orthogonal
8)1,1,1,1,1,1,1,1()1,1,1,1,1,1,1,1()1,W(shiftW 8,38,2 =−−−−⋅−−−−=⋅
Slides Prepared by:Dr. Ivica Kostanic
⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥⎥
⎦
⎤
⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢⎢
⎣
⎡
=
1001011000111100010110101111000001100110110011001010101000000000
8W
Walsh Matrix - Example o A and B use the following Walsh Code
o Assuming A transmits 1 and B does not transmit, what will be the output at the receiver? (+8)
o Assuming A transmits 0 and B does not transmit, what will be the output at the receiver? (-8)
o Assuming A and B transmit 1 with equal power, what will the receiver get?
W(8,3) = -1,1,-1, 1, -1,1,-1, 1 W(8,2) = -1, -1,1,1, -1, -1,1,1
A sent a 1 Code For A
Added:
IEEE 802.11a o Based on IEEE 802.11a standard, rating 54 megabytes
per second (Mbps) n Alternatively called: WiFi5 n 5-GHz radio spectrum
o Range of about 150+ feet (bandwidth decreases with the range – similar to 802.11b) n 802.11a: This is called Data Rate Selection (DRS)
o As the distance changes BW changes from 54,24,9,etc. n 802.11b: DRS will be 11,5.5,2, and finally 1 Mbps as the
distance changes and power reduces o Provides rates of 6, 9 , 12, 18, 24, 36, 48, 54 Mbps
IEEE 802.11a o Uses orthogonal frequency division multiplexing
(OFDM) as the modulation technique o Major issues due to operating at higher
frequencies n Higher path loss n Travels shorter distance (more APs are required)
o Better for indoor applications
IEEE 802.11a – Channel Structure (OFDM)
UNII=Universal Networking Information Infrastructure UNII-1 band à 5.15 to 5.25 GHz UNII-2 band à 5.25 to 5.35 GHz
Total of 12 channels across the spectrum (between all UNIIs: Low, Med, High)
Each UNII has four non-overlapping channels
dBr: dB relative to maximum spectral Density Defined by 802.11h
UNII-1 UNII-2
UNII-3
Max. Spectral Density
802.11a @ Channel 40 (5190MHz-5210 – 20MHz spacing)
Carrier 2 @ UNII-1 in 200 MHz/200 MHz spacing
Compare: 802.11b Spectrum Using Analyzer 3.4
Channel 6 @ 2436; Note there are only 3 overlapping channels
2.4 GHz
OFDM - Basics o Frequency-selective channel is
divided into flat fading subchannels o Fast serial data stream is transformed
into slow parallel data streams n Longer symbol durations
OFDM Advantages o Used in 802.11a o Frequency selective fading affects
some subchannels not the entire signal
o Overcomes intersymbol interference (ISI) in a multipath environment n Each subchannel has much larger period n Thus, time shift results in less ISI
o One major issue with OFDM is ICI n Inter-carrier interference (ICI) – due
to frequency shift n Caused by Doppler effect OR lack of
synchronization o One approach to reduce ICI or ISI is
to use Guard time (Ref: GAST)
Time Domain
Frequency Domain When one signal is at its peak, others are zero
http://www.dsptutor.freeuk.com/analyser/SA102.html
OFDM - Basics o Orthogonal frequency division multiplexing (OFDM)
n Also called multicarrier modulation n Encoding single transmission into multiple subcarriers n Converting fast transmission into smaller transmission n Note: different from CDMA: multiple transmissions into a single
subcarrier! n Also used in 1G mobile phones n FDM has high overhead
o Advantage n Spectral efficiency n Simple implementation n Tolerant to ISI
o Disadvantages n BW loss due guard time n Prone to frequency and phase offset errors n Peak to average power - problem
Spectrum Representation with 3 Channels
OFDM - Basics o Similar to FDM but all frequency channels are dedicated to a single
data source n OFDM uses parallel data and FDM with overlapping subchannels
o Subchannels overlap on each other n Sinc -shaped spectra
o In OFDM the carriers are arranged so that the sidebands of the individual carriers overlap and the signals are still received without adjacent carrier interference.
Single Subchannel OFDM Spectrum
Sinc shape – sidebands overlap
IEEE 802.11a - OFDM o A modulating technique
n Chopping large frequency into subcarrieres
n Some of the bits are transmitted on each subcarrier
o 802.11a n Consists of multiple subcarriers and
base frequency of 0.3125 MHz n Total of 48 channels n Spacing between channels cannot
be less than 15 MHz (48 x 0.3125) n Center frequency will be about
5MHz o The new slower frequencies are all
orthogonal to each other n Independent frequencies
Bit duration=R Center freq = fo Base freq=fb BW=Nfb Period=Ts
Subcarriers Period=NTs Data rate=R/N
Read: http://www.comlab.hut.fi/opetus/333/2004_2005_slides/ofdm.pdf
OFDM Variations o ERP-OFDM (extended rate physical layer) o Flash OFDM - also called fast-hopped OFDM, which uses multiple tones
and fast hopping to spread signals over a given spectrum band. o WOFDM - Wideband OFDM o MIMO-OFDM
IEEE 802.11n – MIMO-OFDM
o Multiple Input, Multiple Output Orthogonal Frequency Division Multiplexing
o MIMO-OFDM is used for WiFi, WiMax and 4G communication systems n Based on the draft IEEE 802.11n standard
o The basic idea is using multiple antennas to transmit and receive radio signals
o The MIMO system uses multiple antennas to simultaneously transmit data, in small pieces to the receiver, which can process the data flows and put them back together. n This process, called spatial multiplexing n It boosts the data-transmission speed by a factor equal to the number of
transmitting antennas
http://www.xcad.com/xcad/mimo.htm
Comparing IEEE 802.11a and b/g o 11a utilizes more available BW o 11a provides higher data rate than 11b and the same as
11g o 11a uses 5GHz which is less cluttered frequency band o Important: 802.11b and 802.11a are not interoperable
or compatible
References o Read about OFDM:
http://www.create.ucsb.edu/ATON/01.01/OFDM.pdf
Security and Authentication
802.11 Security Features o Wireless security can be implemented
at various levels o Privacy
n Defined by Wired Equivalent Privacy (WEP) algorithm o Using RC4 Encrypted algorithm (40-bit
key or 104-bit key)
o Authentication n The end users share a secret key
o General weaknesses n Heavy reuse of shared-key n No key management within the
protocol
WEP Privacy o Contains major weaknesses o Initially 40-bit key - very inadequate o Later 104-bit key – still vulnerable
BASIC PROBLEM: To determine a 104 bit WEP key, we capture between 2000 and 4000 interesting packets…in just a few days!
RC4 o The most widely-used software
stream cipher n Secure Sockets Layer (SSL)
o Protecting Internet traffic n WEP
o Securing wireless networks
Breaking WEP: http://eprint.iacr.org/2007/120.pdf http://www.cdc.informatik.tu-darmstadt.de/aircrack-ptw/
Basic Stream Cipher o A variable key-size stream cipher, typically, with byte orientation o The algorithm was actually revealed in 1994 o Very effective and simple o Basic idea stream cipher
n Bit-by-bit or byte-by-byte n A key is an input to a pseudorandom stream n The output of the generator is called keystream
Keystream
XOR 11001100
01101100
10100000
01101100
11001100
11001100 01101100 XOR -------------- 10100000
11011101 11011101
10100000 01101100 XOR
-------------- 11001100
802.11i Security Features o 802.11i was developed to address security
standards in WLAN o WiFi Alliance Promulgated WiFi-Protected
Access (WPA) n Accepting basic security measures in 11i before
converting to 11i
802.11i Architecture o Data transfer privacy
n MAC layer encryption n Makes sure the data is not altered
o Authentication n Developing a protocol to ensure exchange between
STA and AP using a temporary key over the wireless link
n Examples o EAP: Extensive Authentication Protocol o RADIUS: Remote Authentication Dial-In Service
o Access control (key management) n Makes sure the key exchange is done properly n Regardless of what the actual authentication
technique is
802.11i Basic Architecture o Authentication
n Requires Authentication Server (AS) o AS passes the secret key to the AP o Eventually the key is passed to stations o Station used the key to encrypt the data
between the AP and STA n Offers several encryption techniques
o Uses Advanced Encryption Standard (AES) – 128-bit keys (expensive!)
o 104-bit RC4; compatible with many existing equipments
802.11i Security Features – Data transfer privacy
o MAC level encryption algorithm o Two schemes
n Temporal Key Integrity Protocol (TKIP or WPA-1) o Requires only software changes to devices with WEP
o Uses RC4 104-bit (similar to WEP) encryption algorithm n Counter-mode CBC MAC Protocol (CCMP or WPA-2)
o Uses Advanced Encryption Standard (ASE)
Original MAC
FCS: Frame Check Sequence
802.11i Security Features – Data transfer privacy
24-bit Initialization Vector (IV)- is combined with the 104-bit RC4 key Extended IV (EIV) provides 48-bit IV; thus IV+EIV+RC4 key à 128-bit Encryption key IV and EIV also act as sequence counter à out-f-seq. frames are ignored Original MAC
A CRC calculated on all the fields and then encrypted using RC4 key Used to ensure the message is not modified 64-bit value calculated
using MAC src/des and data fields – uses a separate RC4-based key
No ICV; Uses Advanced Encryption Standard (ASE)
Calculating CRC o Cyclic Redundancy Check o Basic error checking technique
n Parity check n Checksum n CRC
o Basic Operation: n convert binary message to a polynomial n divide it by another predefined polynomial called
the key n The remainder from this division is the CRC n Transmit CRC and Message together
CRC Example (Read page 197) o Assume the message is 11010011101100 o Polynomial is 110101(six bits) o Then, divide Message+00000 by the
Polynomial à n Remainder will be 01110
o Append: 11010011101100 & 01110 n Transmit the message
o The receiver takes 11010011101100 n Divides it by the Polynomial 110101(six bits) n If the message is not altered à there will be no
remainder
Message Authentication Using 1-Way Hashing (P. 402) o We use a hash function to transmit the message
n Message + hashed code is transmitted o Hashing
n mathematical function that converts a large amount of data into a small datum, usually a single integer
o Basic Idea n Compute H(Sab||M) n Transmit M|| H(Sab||M)
o Hash function properties n Weak hash function n Strong hash function
o A Widely used Hash function is MD5
Example: http://www.fileformat.info/tool/hash.htm?hex=10101010000011100000
Questions o How does 802.11i handles
authentication for an IBSS?
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